Machines such as automobiles and machine tools have become more sophisticated in functionality in recent years, and performance required for a lubricating oil to be used in such machines has also become more sophisticated. Although various functions and effects are required for the lubricating oil, an extremely high degree of performance concerning wear prevention has been required for the lubricating oil because the speeds and pressures of the machines have been increasing. An anti-wear agent is typically added as an additive to the lubricating oil for the wear prevention. Such anti-wear agent is an additive that has been well known from long past, and a phosphorus- or sulfur-based compound, or a combination of these compounds has generally been used for the wear prevention (see, for example, Patent Documents 1 to 3).
For example, Patent Document 1 discloses a diesel engine oil for an engine with an exhaust gas recirculation apparatus, the oil being characterised in that a lubricating oil base oil (a mineral oil or a synthetic oil) is blended with 5.8 to 8.3 mass % of a calcium alkyl salicylate (6.0 mass % of calcium (Ca) content) having a total basic number (TBN) of 165 mg KOH/g as a detergent, 0.09 to 0.13 mass % in terms of sine (Zn) of a primary alkyl-type zinc dithiophosphate as an antioxidant-cum-anti-wear agent, and 0.02 to 0.04 mass % in terms of molybdenum (Mo) of an oil-soluble oxymolybdenum dialkyl dithiophosphate as a friction modifier-cum-anti-wear agent.
In addition, Patent Document 2 discloses an anti-wear agent for a low-phosphorus lubricant formed of a composition having the following chemical structure:
[where R and R′ may each represent hydrogen or an alkyl group, and in this case, at least one of R and R′ represents an alkyl group, and R″ represents an alkyl group, R′″OCOCH2, or R′″OCOCH2CH2 (where R′″ represents an alkyl group and X represents S)].
Further, Patent Document 3 discloses a super tractor oil universal lubricating composition characterized in that:
(a) the lubricating composition contains an oil of lubricating viscosity having a viscosity index of at least about 95 and blending additive components containing (i) at least one metal detergent, (ii) at least one phosphorus-based anti-wear agent, and (iii) at least one oil-soluble molybdenum compound;(b) the ratio between a metal content (ppm) based on the total weight of the lubricating composition and the total basic number (mg KOH/g) of the lubricating composition is about 210 to about 450 (ppm/mg KOH/g);(c) the ratio between the metal content (ppm) based on the total weight of the lubricating oil composition and a phosphorus content (ppm) based on the total weight of the lubricating composition is about 5.0 to about 20.0 (ppm/ppm); and(d) the ratio between the phosphorus content (ppm) based on the total weight of the lubricating composition and a molybdenum content (ppm) based on the total weight of the lubricating composition is about 0.5 to about 80.0 (ppm/ppm).
In addition, the applicant of the present application has already proposed that the use of a condensed phosphate can exert a higher wear-preventing effect than that of a con vent tonally known phosphorus-based anti-wear agent (Japanese Patent Application No. 2010-21022).